Sump Pump Generator Size Calculator (by HP)

A typical 1/3 HP sump pump needs about 800W running and 1,300W starting. With 25% headroom that is 1,625W — a 2,000W inverter generator covers it, and the 3,500–4,500W class covers it alongside the refrigerator and lights, which is the realistic storm configuration. The 1/2 HP class (1,050W running, 2,150W starting) pushes a bare 2,000W unit to its surge limit; size up if that is your pump.

They solve different failure modes, and serious basements use both. A battery backup pump (a second pump on a deep-cycle battery) takes over instantly, unattended, at 3 a.m. — but typically pumps at reduced capacity for a limited number of hours. A generator runs the full-power main pump indefinitely but needs a human to deploy it. Battery for the first hours and small storms; generator for the long outage. If choosing one and your area gets multi-day outages, the generator protects more than just the sump.

Cycling duty is kinder on fuel than it sounds: even at a punishing one 30-second run per minute, a 1/3 HP pump averages ~400W — about 10kWh over 24 hours, or very roughly 2–3 gallons of gasoline on a small inverter. The catch is you must be there to refuel; storms that flood basements often last longer than a tank. A dual-fuel unit on a 20 lb propane cylinder buys 8+ hours per bottle unattended by comparison.

Yes — the generator must be 20+ feet out and the pump is in the basement, so a long cord is the design, not a compromise. Make it 12-gauge minimum (10-gauge past 100 ft): every restart pulls the 1,300W surge through that cord, and voltage drop turns easy starts into stalls. Route it through a window with a board-and-notch spacer rather than pinching it in a door, and keep the connection ends out of any water path.

Any 2,000W-class inverter or portable clears the 1,625W recommendation for a single 1/3 HP pump — the harder question is multi-day duty, not peak watts. A hurricane outage that runs 3–5 days favors a dual-fuel unit (gasoline plus 20 lb propane cylinders, which don’t degrade in storage the way old gas does) or a generator with a larger fuel tank and an extended-run kit, because you will be refueling every several hours for days, not running one storm-night tank. Pair the pump with the fridge and lights on the 3,500–4,500W class and the same unit covers the whole outage, not just the basement.

Weeks, not days. Generators sell out at hardware stores in the 48 hours before a named storm’s forecast track firms up, and a unit bought in a panic gets zero test runs before it has to work at 2 a.m. in the rain. Buy before the season’s peak (roughly early-to-mid September in the Atlantic basin), run the pump on it once under normal conditions, confirm the cord length and 12-gauge rating reach from your generator’s safe 20+ ft spot to the basement, and store fuel or propane now — supply chains for both tighten fastest right before landfall, not after.